1. Field
The present invention relates to the fields of digital filtering and analog-to-digital conversion.
2. Description of the Related Art
Generally, the sampling rate that is required to sample an analog signal for A/D conversion must be twice the highest frequency component being sampled. This rate is known as the Nyquist rate. More recently, oversampling methods have been utilized for A/D and D/A conversion. In an oversampling type of convertor, the sampling rate is much higher than the Nyquist rate.
With converters operating under the Nyquist rate for sampling, a certain amount of precision is required for the conversion. For example, in converting an analog signal into a 16-bit digital format, 16-bit precision is required. Accordingly, circuits will need to be designed having components which will meet this precision. In many instances, closely trimmed circuit components or precision matching (or compensating) circuits are required to meet the precision.
However, when sampling at a rate much higher than the required Nyquist sampling rate, the oversampling technique permits circuit precision to be relaxed significantly. For example, an A/D oversampling converter implements an oversampling modulator, and the modulator output can be a single bit output. Accordingly, the circuit precision needs only to meet this 1-bit output, and closely trimmed circuit components are generally not needed. Additionally, 1-bit precision can be readily met by current CMOS (complementary-metal-oxide-semiconductor) components.
If the oversampling technique is adopted, the output generally needs to be reduced at the eventual output of the converter. That is, the higher sampling rate is usually returned to the Nyquist rate. In the above example, a multiple number of the 1-bit outputs will need to be combined to form a single output (e.g., 16-bit, 32-bit, etc.). The oversampling technique is preferred in many applications, since the cost savings in using less precise circuit components outweigh the additional digital signal processing needed at the back end of the converter.
One type of oversampling A/D conversion uses a modulator commonly referred to as a delta-sigma modulator. Delta-sigma modulation is a method for encoding high-resolution signals into lower-resolution signals using pulse-density modulation. In an A/D converter (ADC) using a delta-sigma modulator, an integrator and a comparator are utilized at the front end of the converter to provide the quantization of the analog signal. Then, a digital filter is utilized for digital signal processing to provide a corresponding digital signal at the Nyquist rate.
Conventionally, the digital filter includes several accumulators connected in series. If the number of accumulators is N, this series connection of N accumulators is referenced to as Nth order cascaded accumulators. This results in a large size, and consumes a large amount of chip real estate. When applied to certain devices, such as an image sensor, the consumption is magnified because each column of pixels includes at least one ADC having the digital filter.